This invention relates to reciprocating internal combustion engines, more specifically to two-stroke crankcase scavenged engines. Further, the invention relates to an uncooled crankcase scavenged two-stroke internal combustion engine having a non-lubricated ringless piston.
Well-designed conventional reciprocating engines operating at conservative powers and speeds, irrespective of size or type, generally last about 5.times.10.sup.8 revolutions before wear-out of the piston rings and the cylinder bore. Note that the engine like expectancy is a function of the number of revolutions rather than the total piston travel distance. Such a fact implies that wear-out is primarily due to the ring reversal event, i.e., the reversal in direction of ring travel that occurs twice each revolution. It is well known that maximum bore and ring wear occurs at ring reversal due to the "squeeze-out" of the oil film between the ring and bore when the ring is not moving.
Also, higher piston speed increases wear rates because of the increasing difficulty in maintaining an adequate oil film. It is generally accepted that average piston speed should not exceed about 2000 feet per minute, and low speed engines enjoy a life advantage over high speed engines.
The instant invention prevents any ring/bore wear by removing the piston rings and thus eliminating the contact between the piston and the bore. The resulting leakage of gas around the ringless piston requires corrective measures which are addressed in the instant invention.
The average continuous leakage rate of gas past the piston is about constant irrespective of engine speed. The power and efficiency loss due to gas leakage past the piston is directly proportional to the weight of gas leaked each engine cycle. Therefore, the higher the engine rotational speed, the less gas is leaked each engine cycle, and the less the power and efficiency loss, even though the average continuous leakage rate is high.
In the conventional four-stroke engine, any gases leaked past the piston enter directly into the engine crankcase and contaminate the engine lubricant. At high leakage rates the lubricant is blown out of the crankcase. In the present invention, gas leakage rates are high so it is not feasible to allow these gases to pass directly to the crankcase. However, in a two-stroke engine that uses the underside of the piston to pump air to scavenge the cylinder, any gases leaked past the piston will return to the cylinder via the transfer port without the possibility of lubricant contamination or the necessity for disposal of the leaked gases to the atmosphere.
In conventional engines, piston speed is limited to about 2000 feet per minute. The engine of the instant invention has no such limits. Therefore, it is possible to run the engine very fast, as is required in the present invention. Further, it is desirable to run the instant engine very fast in order to increase the horsepower output without recourse to turbocharging or supercharging.